When an appropriate gray level voltage is applied to a pixel of a TFT LCD panel, the angle of liquid crystal molecule in the pixel changes correspondingly. This angle change further alters transmittance of the TFT-LCD panel so a desired gray level can be achieved. However, due to the intrinsic property of liquid crystal molecule, if the gray level has to change dramatically during two successive refresh periods, the desired angle change may not be achieved in one refresh period. This results in a blurred display, and the situation is particularly bad for a motion picture display.
One solution to this problem is to use an over-drive technique. The over-drive technique applies a gray level voltage (over-drive gray level voltage) higher than originally required, so the angle of liquid crystal is changed from the initial gray level to the target gray level in a refresh period. The relationship between the initial gray level voltage, the target gray level voltage, and the over-drive gray level voltage can be obtained from a Look-Up Table. The Look-Up Table is a table providing the corresponding over-drive gray level voltage when the pixel has to change from an initial gray level voltage to a target gray level voltage. FIG. 1 shows a Look-Up Table of an 8-bits driving system. The horizontal axis represents the initial gray level voltage, and the vertical axis represents the target gray level voltage. The intersection is the over-drive gray level voltage applied to the pixel. For example, if the initial gray level voltage is V32, and the target gray level voltage is V64, the over-drive gray level voltage applied to the pixel would be V80.
FIG. 2 is a block diagram showing a conventional driving system utilizing the over-drive technique. Timing controller 201 retrieves Gn frame image data from an image data source, and retrieves previous Gn−1 frame image data from a frame buffer 202. Timing controller 201 then compares the Gn and Gn−1 frame image data and addresses the pixels that need to be updated. Subsequently, timing controller 201 retrieves the Look-Up Table stored in a memory 203, and converts the image data in the updated pixels to a corresponding over-drive gray level voltage. The over-drive gray level voltage is then applied to the pixel via a source driver.
However, the driving system utilizing the over-drive technique still has some drawbacks. First, only the pixels where image data has to change during the two successive refresh periods is updated. This requires several frame buffers to store the previous frame image data in order to compare the image data in the same pixel during the two successive refresh periods. However, frame buffers are expensive and dramatically increase the manufacturing cost. Besides, the Look-Up Table utilized in the over-drive technique records the increment, and SRAM needs to be put in the timing controller, so the design of the circuit is complicated. Furthermore, the chip size is bigger and the power consumption thereof is higher. On the other hand, the pictures with high gray level are saturated, and the color depth is thus affected.